Process for the removal of acid forming gases from exhaust gases and production of phosphoric acid

ABSTRACT

Exhaust gases are treated to remove NO or NO x  and SO 2  by contacting the gases with an aqueous emulsion or suspension of yellow phosphorous preferably in a wet scrubber. The addition of yellow phosphorous in the system induces the production of O 3  which subsequently oxidizes NO to NO 2 . The resulting NO 2  dissolves readily and can be reduced to form ammonium ions by dissolved SO 2  under appropriate conditions. In a 20 acfm system, yellow phosphorous is oxidized to yield P 2  O 5  which picks up water to form H 3  PO 4  mists and can be collected as a valuable product. The pressure is not critical, and ambient pressures are used. Hot water temperatures are best, but economics suggest about 50° C. The amount of yellow phosphorus used will vary with the composition of the exhaust gas, less than 3% for small concentrations of NO, and 10% or higher for concentrations above say 1000 ppm. Similarly, the pH will vary with the composition being treated, and it is adjusted with a suitable alkali. For mixtures of NO x  and SO 2 , alkalis that are used for flue gas desulfurization are preferred. With this process, better than 90% of SO 2  and NO in simulated flue gas can be removed. Stoichiometric ratios (P/NO) ranging between 0.6 and 1.5 were obtained.

The U.S. Government has rights in this invention pursuant to ContractNo. DE-AC03-76SF00098 awarded by the U.S. Department of Energy to theUniversity of California.

This application is a continuation-in-part of co-pending applicationSer. No. 261,229, filed Oct. 24, 1988 now allowed.

BACKGROUND OF THE INVENTION

The present invention relates to an improved process for the removal ofacid gases including NO_(x) from exhaust gases, particularly to acombined removal of NO_(x) and SO₂ from flue gas and the like and alsoto the acquisition of valuable products from the process. (Flue gasusually contains both nitric oxide (NO) and nitric dioxide (NO₂); theseoxides of nitrogen are collectively given as NO_(x).)

Concerns about air pollution caused by acid rain are increasing worldwide, and considerable research effort is being expended to provideeffective treatment of flue gases and other exhaust gases to remove acidforming components therefrom. However, the present methods havedisadvantages which are particularly acute with respect to the removalof NO_(x). In addition, the present methods are extremely costly.

Early methods were primarily used to remove pollutants when theconcentrations were very high. As time goes by, and larger volumes ofgases are generated, tolerable levels of emissions keep getting lowerand lower. At this time emissions may be treated to obtain acceptablelevels of SO₂ by means of scrubbing processes using aqueous solutions.However, removal of NO_(x) presents problems, the most serious beingsufficient removal and economic considerations. In addition, theeconomics of using two processes has prompted efforts to utilize wetscrubbing for removal of both NO_(x) and SO₂ in a single process, andsome success has been achieved in this direction. Due to the difficultyin solubilizing NO in aqueous solution, these processes have utilizedexpensive ingredients and often have provided other products requiringdisposal.

Wet processes developed for removal of NO_(x) have been reported. Forexample, Patent No. P 32 38 424.6 issued by the Federal Republic ofGermany Apr. 19, 1984 to Hoechst AG utilizes red phosphorus in inertoxidizing media to remove NO and NO₂ from flue gas. However, the patentreports the treatment of very high concentrations of NO, typicalconcentrations being up in the thousands of parts per million, and inExample 7 of the patent where 1000 parts per million were treated, only40% was removed. In the two part Example 9, the patentee reports 14,000parts per million were treated in the first step to obtain a 90% removalto 1,300 parts per million; and in the second part about a 65% removalto about 460 parts per million. Such effluent concentrations are notsufficiently low enough, and we have found that red phosphorus is notsatisfactory to treat concentrations of 500 parts per million or less.

Current NO_(x) standards for power plant emissions may be attainableusing the selective catalytic reduction (SCR) process which is veryexpensive. In addition, there is very limited experience with SCR on UScoal with high sulfur content and variable ash composition. High SO₂concentration promotes the formation of ammonium sulfate/bisulfateparticulates, which result in the plugging of air heaters of boilers.Ash composition rich in arsenic and alkali could be detrimental tocatalysts employed in the SCR system. Other approaches for thereductions to amounts less than 100 ppm are reported in U.S. Pat. No.4,079,118 entitled Method for Removing Nitrogen Oxides Using Ferricion-EDTA Complex Solutions issued Mar. 14, 1978, and various other wetprocesses have been developed to provide efficient removal of NO_(x).However, these processes generally require either the use of expensivestarting materials or create a disposal problem for the products of theprocesses or both.

Numerous other patents have been issued which disclose wet processes forremoval of NO_(x) such as U.S. Pat. No. 3,984,522; U.S. Pat. No.4,079,118 and U.S. Pat. No. 4,158,044. In addition, many patents haveissued which disclose combined processes for removal of both SO₂ andNO_(x). Examples of such patents include U.S. Pat. Nos. 4,126,529 and4,347,227. Many other systems have been suggested, and the list is toolong to include them all. However, there is much room for improvement inproviding a practical, efficient removal process for both of suchpollutants either individually or together.

As mentioned above, sulfur oxides can be effectively removed by flue gasdesulfurization scrubbers. The majority of these scrubbers now in useinvolve wet limestone processes, which utilize aqueous slurries oflimestone to neutralize the sulfurous and/or sulfuric acids producedfrom the dissolution and subsequent oxidation of flue gas SO₂ inscrubbing liquors. The resulting solid slurries, containing CaSO₃.1/2H₂O and gypsum (CaSO₄.2H₂ O), can be hauled away for disposal. Suchpractice is common among power plants located in areas where landfillspace is abundant. On the other hand, the more practical solution forpower plants situated in densely populated areas is to operate thescrubbers under forced oxidation conditions. Under those circumstances,the major by-product of the scrubbing process is gypsum, which is ofsome commercial value as a building material.

Further versatility in the processing by flue gas desulfurizationscrubbers is obtained by utilizing other alkalis besides limestone orlime. These include soda ash (Na₂ CO₃), nahcolite (NaHCO₃), trona (Na₂CO₃ /NaHCO₃), Na₂ SO₃, NaOH, KOH, K₂ CO₃ /KHCO₃, magnesite (MgCO₃),dolomite (CaCO₃ /MgCO₃), NH₄ OH, and (NH₄)₂ CO₃ /NH₄ HCO₃. Thesematerials are more expensive than limestone and are more often used inchemical industries where the volume of waste gas to be treated is smallcompared to those from power plants, and where the plants are in closeproximity to the production sites of those alkalis.

While the wet flue gas desulfurization scrubbers described above arevery efficient in the removal of SO₂ from flue gas, they are incapableof removing sufficient NO because of its low solubility in aqueoussolution. NO makes up about 95% of the NO_(x) in most flue or exhaustgases. The installation of a separate scrubber for flue gasdenitrification generally requires additional capital investment.Accordingly, approaches to modify existing wet flue gas desulfurizationprocesses for the simultaneous removal of SO₂ and NO_(x) emissions havebeen under world wide investigation.

Several methods have been developed to enhance the absorption of NO_(x)in scrubbing liquors. These include the oxidation of NO to the moresoluble NO₂ using oxidants such as O₃, ClO₂, and KMnO₄, as well as theaddition of various iron(II) chelates to the scrubbing liquors to bindand activate NO (See, H. I. Faucett, J. D. Maxell and T. A. Burnett,"Technical Assessment of NO_(x) Removal Process for UtilityApplication", EPRI AF-568. EPA600/7-77-127 March, 1978). So far, none ofthese methods has been demonstrated to be cost effective, despite highremoval efficiencies of both SO₂ and NO_(x).

SUMMARY OF THE INVENTION

A primary object of this invention is to provide a wet scrubbing processwherein NO_(x) may be removed from exhaust gases such as flue gas to adegree that the remaining concentrations may be lower than 200 parts permillion, and, if desired, lower than 10 to 20 parts per million byvolume.

This objective is achieved by a method of treating exhaust gasescontaining NO_(x) comprising the step of contacting the exhaust gas withan aqueous emulsion or suspension containing yellow phosphorus (P₄). Thecontact of the exhaust gas may be by any suitable contact method such asin a spray type or a bubbling type absorber. At least some oxygen oroxygen source must be present in the exhaust gases, and most exhaustgases contain a sufficient amount; however, air or other sources ofoxygen may be added to the exhaust gas when needed or wanted. Thepressure is not critical, and the process is generally carried out atambient or such positive pressures needed to move the gases through ascrubber.

The temperature of the process is operative throughout the liquid rangefor water, and optimally in the range of about 20° C. to about 95° C.,with a preferred range of about 45° C. to 75° C. The concentration ofyellow phosphorus (also known as white phosphorus) required is ratherlow because any amount is functional, but it should be above about 0.01%by weight in the aqueous emulsion or suspension and best above 0.1%. Thehigh level would be any amount that allows for enough water to carry outthe reactions and provide the desired safety conditions, and could be20% or even higher at the front end of the contacting apparatus. Apreferred range would be about 0.1 to 10.0% by weight in order to obtaingood results, and optimally from 0.2 to 3% by weight. The pH may alsovary over a large range up to a pH of 9 and any pH under 9 appears to beoperative for purposes of oxidizing the P₄ . However, for removing (i.e.absorbing) NO_(x) and also other materials such as SO₂ the pH should beabout 3.0 or above, in the range of about 3.0-9.0, preferably 3.0-7.0. ApH higher than 9.0 can give undesirable by-products.

Another primary object of the invention is to produce a phosphoric acid(H₃ PO₄) product during the NO_(x) removal process.

In this process the yellow phosphorus oxidizes to P₄ O₁₀, commonlycalled phosphorus pentaoxide or P₂ O₅, during contact of the phosphorusaqueous emulsion or suspension with oxygen present in the exhaust gases.The P₄ O₁₀ then associates with water droplets or water vapor to becomephosphoric acid in the form of a white smoke or phosphoric acid mistwhich can be collected by various methods. The minimum contact time ofthe phosphorus emulsion or suspension with the exhaust gases should beabout 0.5 seconds for a spray type scrubber and about 0.05 seconds in abubbling type scrubber. When the pH of the emulsion or suspension isabout 3 or greater, the scrubber becomes an absorber of NO₂ and SO₂ andthe duration of the contact time becomes critical in that too long acontact time will result in the P₄ being converted to phosphates. Forthe purpose of recovering phosphoric acid the contact time should be nolonger than about 10 seconds in a spray type absorber and about 3.5seconds in a bubbling type absorber.

Another object of the invention is to provide a process wherein bothNO_(x) and SO₂ are removed in a single process using an apparatus whichis now conventional in SO₂ removal processes, and wherein valuableby-products are obtained.

As indicated above, the yellow phosphorus emulsions or suspensions areespecially suitable when the pH is adjusted to within the range of about0.0 to about 9.0, and such adjustment, if needed, may be made by usingany suitable alkaline material. When the emulsion is kept alkaline, NO₂and sulfur dioxides are also removed. By using limestone, or one of thealkalis mentioned in the background section above for flue gasdesulfurization, the advantages of such processes are obtained alongwith removal of NO_(x). The resulting by-products could thereforeinclude phosphate, nitrate, and sulfate salts of calcium, magnesium,sodium, potassium and ammonium. These products are important nutrientsfor plants and constitute the major components of fertilizers.

Still another object of the invention is for phosphoric acid to be theby-product in the NO_(x) and SO₂ removal process which uses an apparatusconventional in SO₂ removal processes.

A further object of the invention is the provision of a process forremoving NO_(x) and SO₂ from flue gas and the like which is capable oftaking out substantial amounts of the NO_(x) (about 20%-95%) and SO₂(about 40%-98%) from the flue gas, and which also provides suitableby-products from the process.

Further objects and advantages will be apparent as the specificationproceeds and the preferred embodiments are described in detail.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates data in graphical form obtained from a group ofexperiments wherein NO is removed from a simulated flue gas usingdifferent amounts of yellow phosphorus.

FIG. 2 shows data in graphical form obtained from a group of experimentswherein NO is removed from a simulated flue gas using yellow phosphorusemulsions at various temperatures.

FIG. 3 shows data in graphical form obtained from a group of experimentswherein NO is removed from a simulated flue gas using an aqueousemulsion of yellow phosphorus at various pH conditions.

FIG. 4 shows data in graphical form obtained from a group of experimentswherein NO is removed from a simulated flue gas using yellow phosphorusand wherein the amount of oxygen in the flue gas is varied.

FIG. 5 shows data in graphical form obtained from an experiment whereinboth NO and SO₂ are removed from a simulated flue gas using a mixture ofan aqueous emulsion of yellow phosphorus and a slurry of limestone.

FIG. 6 is a schematic diagram illustrating a 20 acfm bench-scale wetphosphorus/limestone process using a spray tower scrubber.

FIG. 7 is a schematic diagram illustrating a bench-scale Bechtel CT-121type scrubber consisting of a spray tower prescrubber and a bubblingabsorber.

FIG. 8 shows data in graphical form obtained from an experiment whereinNO is removed from a simulated flue gas over a period of about threehours using a yellow phosphorus emulsion in a prescrubber and alimestone slurry in a bubbling absorber. The P/NO ratio determined forthis run was 0.73.

FIG. 9 shows data in graphical form obtained from a group of experimentswherein NO is oxidized and removed from a simulated flue gas in a spraytower scrubber wherein the flow rate of the simulated flue gascontaining 310 ppm NO, 2000 ppm SO₂ and 5% O₂ is varied.

FIG. 10 shows data in graphical form obtained from a group ofexperiments wherein NO is oxidized in a spray tower prescrubber andremoved in a bubbling absorber type scrubber.

FIG. 11 shows data in graphical form obtained from a group ofexperiments wherein NO is oxidized and removed from a simulated flue gasusing a yellow phosphorus and limestone aqueous mixture in a spray towerscrubber and the contact time is varied.

FIG. 12 shows data in graphical form obtained from a group ofexperiments wherein NO is oxidized using yellow phosphorus in a CT-121type prescrubber and the contact time is varied.

FIG. 13 is a conceptual flow diagram of a wet phosphorus/limestoneprocess for combined removal of SO₂ and NO_(x).

DETAILED DESCRIPTION OF THE INVENTION

Certain fuels are already low in sulfur, and other fuels have beentreated to remove sulfur prior to combustion. In such cases, sulfuroxide pollution is not a serious problem and generally the exhaust gasesare discharged to the atmosphere without treatment. These fuelsfrequently contain nitrogen compounds that appear in the exhaust gas asNO_(x). In addition, NO_(x) may be produced from high temperaturedecomposition of N₂ and O₂ in air during the combustion and it is nowdesired to treat such exhaust gases. For example, power plants usingnatural gas in California emit exhaust gases with the NO_(x)concentration greater than 75 parts per million, while the Californiastandards are being set to require emissions of less than 25 parts permillion NO_(x).

We have discovered that NO_(x) can be removed from flue gas using anaqueous emulsion containing liquid yellow phosphorus. The factorsinfluencing the effectiveness of NO removal of our system relate to theNO oxidation efficiency and include the amount of phosphorus used, thetemperature of the aqueous emulsion, the pH of the aqueous emulsion, thecontact time of the gases with the emulsion and the O₂ concentration inthe flue gas.

As used herein, we express the effectiveness for NO removal of a yellowphosphorus emulsion as the stoichiometric ratio P/NO, which is definedas the number of moles of phosphorus needed to remove one mole of NOaveraged over the entire period of an experiment (usually 2 hours).Therefore, the higher the stoichiometric ratio, the more phosphorus isrequired to remove each mole of NO, the lower the effectiveness for NOremoval and vice versa.

It should be pointed out that the reaction mechanism for NO removal byyellow phosphorus is distinctively different from that using redphosphorus. The reaction between yellow phosphorus and NO appears totake place in both aqueous and gas phase due to the low melting point(44.1° C.) and high vapor pressure of yellow phosphorus. On the otherhand, red phosphorus at atmospheric pressure is solid up to atemperature of about 417° C. (where it sublimes), and therefore has avery low vapor pressure at the reaction temperatures employed herein(about 20° C. to 95° C.). In this case, the absorption of NO is likelyto be solid-gas type. Furthermore, the NO-derived products using yellowphosphorus have been determined to include NO₂ ⁻ and NO₃ ⁻, bothoxidation products of NO, whereas in the case of red phosphorus, it wasclaimed in the Hoechst AG patent (cited above) that N₂, a reductionproduct of NO, was the only nitrogen product obtained. The difference inNO-derived products in these two cases also suggests different reactionmechanisms are involved.

The phosphorus oxidation process may also be directed to the productionof phosphoric acid. The experiments such as those in Examples 1, 2, 3, 4(infra) which removed NO_(x) from simulated flue gas were all carriedout in a 2 inch diameter bubbling absorber column with 200 ml ofreaction mixture. The gas flow rate in these experiments was about3.5×10⁻² acfm (1 liter/min), corresponding to a superficial gas velocityof 0.028 ft/sec in the absorption column and a contact time of flue gaswith scrubbing liquors of approximately 12 sec. However, in a commercialscrubber system, the superficial velocity of flue gas is much faster(8-12 ft/sec), and the contact time much shorter (2-5 sec).Consequently, the mass transfer and chemical reaction kinetics are lessfavorable under the conditions of a commercial system. In order todetermine whether the P₄ additive is still effective with wet limestonesystems at realistic conditions, a 20 acfm bench-scale scrubber systemwas constructed that simulates as close as possible the conditions of acommercial system. Under these conditions, the major oxidation productof P₄ is phosphoric acid. In the phosphorus oxidation process finelydivided phosphorus pentaoxides are generated which pick up moisture toform phosphoric acid aerosols giving the appearance of white smoke.Recovery of the white smoke yields a valuable by-product, phosphoricacid. A preferred method of recovery is to absorb the phosphorus "whitesmoke" with concentrated phosphoric acid (40-75%).

The P/NO ratios determined were in the range of about 1.0-1.5 when abench-scale spray tower was used as the absorber, and about 0.6-1.0 whenthe CT-121 type bubbling configuration was the absorber. The P/NO ratioswere determined from batch runs. A known weight of P₄ was added inwater, then the experiment was carried out until the NO removal reachedzero. By integrating the NO removal curve for the entire period of theexperiment, and knowing the amount of P₄ used, the P/NO ratio wascalculated. A P/NO determination for a constant removal efficiency of NOhas not been performed.

An investigation of factors affecting P/NO is underway. A large amountof O (atomic oxygen) was detected in the reaction zone during thereaction of P₄ with O₂. The reaction is believed to proceed via abranched-chain mechanism. (See, N. N. Semenov. "Die Oxydation desPhosphordampfes bei Niedrigen Drucken." Z. Phys. 46, 109, 1927.)Investigators have identified several elementary reactions involved inthe chain, and have determined rate constants for some of the reactions.However, a complete list of elementary reactions is not yet available.Dainton and Kimberly (See, F. S. Dainton and H. M. Kimberly. "Reactionbetween Phosphorus Vapor and Oxygen." Trans. Faraday Soc. 46, 629,1950.) have proposed the following reaction scheme:

    P.sub.4 +O.sub.2 →P.sub.4 O+O                       (1)

    P.sub.4 O.sub.n +O.sub.2 →P.sub.4 O.sub.n+1 +O      (2)

where n=1,2, . . . 9

The overall reaction is

    P.sub.4 +10 O.sub.2 →P.sub.4 O.sub.10 +10°   (3)

The reaction of O with O₂ forms O₃, in the presence of another moleculeM which remains unchanged after the reaction:

    10°+10 O.sub.2 +10M→10 O.sub.3 +10M          (4)

According to this reaction scheme, each P₄ reacts with 10 O₂ to generate10 O₃. If all the O₃ produced oxidizes NO to NO₂, the P/NO ratio will be0.4, provided the removal of NO occurs by the dissolution of NO₂ (or N₂O₄). The P/NO ratio will be 0.2 if the removal of NO occurs by thedissolution of N₂ O₃. In reality, the dissolution of a mixture of NO₂and N₂ O₃ in the scrubbing liquor is more likely. The O₃ generated maybe consumed by reaction with SO₂ through gas phase as well as liquidphase reactions, which would increase the P/NO ratio. The gas phasereaction SO₂ +O₃ →SO₃ +O₂ is much slower (rate constant k<8×10⁻²⁴ cm³.molecule⁻¹. sec⁻¹ at 20° C.) than NO+O₃ →NO₂ +O₂ (k=1.7×10⁻¹⁴ cm³molecule ⁻¹ sec⁻¹ at 20° C.) and is negligible. (See, J. G. Calvert, F.Su, J. W. Bottenheim, and O. P. Strausz. "Mechanism of the HomogeneousOxidation of Sulfur Dioxide in the Troposphere." Atmos. Environ., 12,197, 1978 and H. S. Johnston, S. G. Chang, and G. Whitten, "Photolysisof Nitric Acid Vapor." J. Phys. Chem., 78, 1, 1974 respectively.) Thereaction of O₃ with HSO₃ ⁻ /SO₃ ²⁻ in liquid phase is fast, but takesplace only after the dissolution of O₃ in scrubbing liquors. Thesolubility of O₃ is small. The Henry's constant of O₃ is 1.23×10⁻²M.atm⁻¹ at 20° C. The concentration of gaseous O₃ produced is related tothe vapor pressure of P₄, which is about 325 ppm at 50° C. (In reality,P₄ concentration is expected to be much smaller because of kineticlimitation. The residence time of spray in a column is short. The P₄evaporation rate from the spray is the rate determining step.) One cancalculate that the concentration of O₃ dissolved in the liquor is only4.0×10⁻⁵ M when in equilibrium with 3250 ppm of O₃, the upper limit in aspray column. Therefore, the dissolved O₃ is only a small fraction (lessthan 1%) of the total O₃ at a typical L/G ratio (60-120). Also, O₃ canbe consumed by P₄ during its oxidation. The rate constants of reactionof O₃ with P₄ and its oxidation derivatives have not been reported. Itis difficult to estimate the fraction of O₃ that would be consumed byphosphorus containing species. However, the reaction rate constants ofP₄ with O₂ is comparable to that of P₄ with O. The concentration of O₂is orders of magnitude larger than O. Most of the P₄ is expected to beoxidized by O₂. Based on the chemistry described, the presence of SO₂ inthe flue gas is probably not going to affect the result significantly.The P/NO ratio can be improved by using good mixing conditions, wherethe P.sub. 4 spray is dispersed uniformly and the O₃ is accessible tothe NO in the flue gas. Also, as stated previously, the temperature, P₄concentration of the spraying liquor, NO and O₂ concentrations in theflue gas, and L/G will influence the P/NO ratio. Furthermore, O₃ and Omay be consumed on the surface of the wall. A large-diameter spraycolumn will reduce this wall effect and improve the effectiveness of P₄utilization.

The fate of P₄, NO, and SO₂ in the system has been studied. The reactionof P₄ with O₂ generated phosphoric acid as white smoke. Theconcentration of white smoke in flue gas appeared to decrease slightlyas the flue gas passed through the absorber but the amount absorbed wasnot a substantial portion (i.e. less than 25%). The analysis of thescrubbing liquor by ion chromatography showed that the liquor containedphosphorus-containing species adding up to only 8-12%, and 15-25% of theP₄ consumed with a spray tower and with a CT-121 type absorber,respectively. The unabsorbed white smoke could be removed from the fluegas by treating it downstream from the absorber with concentratedpreferably 40%-75%, phosphoric acid. The oxidation products of P₄consisted of phosphoric acid (H₃ PO₄), phosphorous acid (H₃ PO₃), andhypophosphorous acid (H₃ PO₂), with their molar ratio roughly in 10, 2,and 0.2, respectively, at the experimental conditions employed.

The analysis of scrubbing liquors revealed the presence ofnitrogen-sulfur compounds, in addition to NO₃ ⁻, HSO₃ ⁻, SO₄ ²⁻, H₂ PO₄⁻, H₂ PO₃ ⁻, and H₂ PO₂ ⁻. Only 5 to 15% of the NO removed was convertedto NO₃. The majority of NO absorbed was found to be converted tonitrogen-sulfur compounds. The nitrogen-sulfur compounds areintermediates produced from the reaction of NO₂ ⁻ with HSO₃ ⁻. Manyconcurrent and consecutive reactions can take place and result in theproduction of intermediates, including hydroxyimidodisulfate [HON(SO₃⁻)₂ ], hydroxysulfamate [HONHSO₃ ⁻ ], hydroxylamine [NH₂ OH],nitridotrisulfate [N(SO₃ ⁻)₃ ], imidodisulfate [HN(SO₃ ⁻)₂ ], andsulfamate [NH₂ SO₃ ⁻ ]. These nitrogen-sulfur intermediates havedifferent reactivities and exhibit different half-lifes in the scrubbingsystem. The steady state concentrations of these intermediates varydepending on the scrubbing conditions. Hydroxyimidodisulfate andimidodisulfate are two intermediates most often found in highconcentrations under the experimental conditions employed. When there isan excess of HSO₃ ⁻ present in the liquor, such as conditionsencountered using flue gas from a high-sulfur coal, thesenitrogen-sulfur compounds are converted eventually to sulfamate ion,which then hydrolyzes to produce NH₄ ⁺ ion in an acidic medium.

    NO.sub.2.sup.- +2HSO.sub.3.sup.- →HON(SO.sub.3).sub.2.sup.2- +OH.sup.-                                                 (5)

    HON(SO.sub.3).sub.2.sup.2- +HSO.sub.3.sup.- →N(SO.sub.3).sub.3.sup.3- +H.sub.2 O               (6)

    N(SO.sub.3).sub.3.sup.3- +H.sub.2 O→HN(SO.sub.3).sub.2.sup.2- +SO.sub.4.sup.2- +H.sup.+                                 (7)

    HN(SO.sub.3).sub.2.sup.2- +H.sub.2 O→H.sub.2 NSO.sub.3.sup.- +SO.sub.4.sup.2- +H.sup.+                                 (8)

    H.sub.2 NSO.sub.3.sup.- +H.sup.+ →H.sub.2 NSO.sub.3 H (9)

    H.sub.2 NSO.sub.3 H+H.sub.2 O→NH.sub.4.sup.+ +SO.sub.4.sup.2- +H.sup.+                                                  (10)

The overall reaction is

    NO.sub.2.sup.- +3HSO.sub.3.sup.- +H.sub.2 O→NH.sub.4.sup.+ +3SO.sub.4.sup.2- +H.sup.+                                (11)

A fraction of the absorbed SO₂ is converted to nitrogen-sulfurintermediates as described above. These intermediates will eventuallydecompose to form SO₄ ²⁻ as the final product of absorbed SO₂.

The invention is illustrated further by the following examples which arenot to be construed as limiting the scope of the invention.

EXAMPLE 1

The removal of NO from flue gas by yellow phosphorus in water wasstudied using a bench scale scrubber. The scrubber was an uprightcylindrical Pyrex column (50 mm i.d.×210 mm) with a fritted disc bottomcapable of holding aqueous liquid. The scrubber was equipped with athermometer for measuring the temperature of liquid therein and a pHelectrode for measuring the pH of the liquid therein. A water jacket wasprovided to heat or cool the contents of the scrubber, and appropriatefeed lines to admit measured amounts of gases for the simulated flue gasare provided. With this set-up accurate amounts of NO, N₂, O₂ and SO₂are provided to the scrubber. The downstream side of the scrubber isequipped with appropriate condensers, an absorber, cold trap andanalyzers for NO_(x) and SO₂. 1.0 gram of yellow phosphorus (m.p.=44.1°C.) was melted in 0.2 liters of water at 60° C. in the scrubber. The pHof the aqueous phase was between 3 and 4. Yellow phosphorus globuleswere dispersed in water upon the bubbling of a gaseous mixturecontaining about 500 ppm NO, from 0 to 20% O₂, and the balance N₂through the bottom of the column at a flow rate of 0.8-1.0 liter perminute. In addition to these runs wherein the amount of O₂ was varied,other runs were made wherein the amount of phosphorus was varied, thetemperature of the aqueous emulsion was varied, and in which the pH ofthe aqueous emulsion was varied.

The gas mixture leaving the reaction column was passed through acondenser (length=390 mm), a gas washing bottle containing 0.2 liters ofa 0.2M NaOH solution, a second condenser (length=200 mm), and then acold trap (-84° C.). The NO and NO₂ concentrations in the outlet gas wasmeasured by a Thermoelectron Model 14A chemiluminescent NO_(x) analyzer.The reaction was stopped after 2 hours. The pH of the scrubbing liquorand the NaOH absorber solution after the experiments were generallyabout 1.5 and 12.5, respectively. The NO and phosphorus derived productsin the spent solution in the scrubber as well as the NaOH absorber weredetermined by ion chromatography.

The passage of the simulated flue gas mixture through the scrubbingcolumn containing the molten phosphorus creates a fine yellow phosphorusdispersion in water. When O₂ is present in the flue gas, a dense whitephosphoric acid fume is produced which could lead to a significantresponse from the chemiluminescent NO_(x) analyzer if left unchecked.This is believed to result from the chemiluminescence produced byincomplete oxidation of phosphorus. This interference decreasedsubstantially when the partial pressure of O₂ in the flue gas isincreased, consistent with the complete oxidation of phosphorus underthose conditions.

The use of a NaOH absorber and a cold trap coupled with the monitoringof the scrubbed flue gas using the NO_(x) mode on the chemiluminescentanalyzer (which involves passage of the gas mixture through a stainlesssteel column at 650° C. half of the time) eliminates the white fumes.

The reaction was carried out using various amounts of phosphorus in theemulsion (at pH 3), and with an O₂ concentration of 4% in the simulatedflue gas. The results of these runs are shown graphically in FIG. 1. Itis clear that the initial NO removal efficiencies were higher at higherconcentrations of phosphorus and reaches about 90% at 2.0% by weightyellow phosphorus.

The effect of the temperature of the emulsion was determined in a numberof experiments, and the results are shown in FIG. 2. In these runs, theemulsion contained 0.5% yellow phosphorus at pH 3 and the flue gascontained 550 ppm NO, 4.0% O₂, and the remainder N₂. Whereas the initialNO removal efficiencies were higher at higher temperatures, the overalleffectiveness for NO removal were lowered under these conditions. Forinstance, the initial removal percentage of NO was increased from 78% to99% when the temperature of the emulsion was raised from 50° C. to 75°C.

The influence of pH on the effectiveness for NO removal of a yellowphosphorus emulsion has been determined, and the results of theexperiments are shown in FIG. 3. In these runs, the O₂ content wasadjusted to 4% by volume. As shown in FIG. 3, the effectiveness for NOremoval increases with increasing acidity of the aqueous phase over thepH range of 3.0 to 9.0.

The influence of O₂ concentration in the flue gas was also determined,and the data is shown in FIG. 4. As there shown, the presence of O₂ isessential for the removal of NO by yellow phosphorus emulsions. Inaddition, the effectiveness for NO removal of a phosphorus emulsionincreases as the O₂ content of the simulated flue gas mixture increases.In these runs, the NO absorption reaction was carried out at pH 3 and60° C. using a 0.5% by weight yellow phosphorus emulsion. The use ofyellow phosphorus for the removal of flue gas works best under forcedoxidation conditions.

Example 2 (comparison example)

A comparison of the NO removal effectiveness of yellow phosphorus andred phosphorus was made for use in treating simulated flue gases having500 ppm NO using the apparatus of Example 1. Both emulsions of 0.5% byweight yellow phosphorus and suspensions of 1.5% by weight redphosphorus were used to treat a simulated flue gas of 500 ppm NO and 4%O₂ at 60° C. The yellow phosphorus emulsion removed up to 80% of the NOwhereas none of the red phosphorus emulsions removed any detectableamount. At pH 9, the yellow phosphorus emulsion removed up to 40% of theNO whereas the red phosphorus still did not remove a detectable amount.At pH 10.1, the red phosphorus did remove some NO but the effectivenesswas still very low (P/NO about 1,000).

Example 3

Spray drying experiments were carried out using a Niro Atomizer portablespray dryer equipped with a Type M-02/a centrifugal atomizer. The volumeof the spray drying chamber was about 350 liters, and the gas flowcapacity was about 500 liters per minute. Yellow phosphorus wasintroduced to the spray dryer chamber either in liquid form (as anemulsion in water) or in solid particulate form (as a fine particulatedispersion in water prepared by the rapid cooling of a phosphorus inwater emulsion from about 80° C. to room temperature). The inlettemperature of the simulated flue gas mixture (containing 490 ppm NO,20% O₂, and the balance N₂) was 170° C. and the exit gas temperature was65° C. Using a 0.25% by weight yellow phosphorus emulsion up to 40% ofthe NO was removed. In a separate experiment, fine particulatedispersions of yellow phosphorus (5% by weight) also containing 3.2Murea were used in the spray drying system. The simulated flue gascontained about 550 ppm NO, and up to about 70% of the NO was removed.It is expected that higher removals may be achieved using a moreconcentrated phosphorus emulsion and/or under better operatingconditions.

Example 4

In this example, various levels of NO in the simulated flue gas weretreated using the apparatus of Example 1. The 150 cc aqueous emulsioncontained 1.0 gram of CaCO₃ in all cases except Example 4f, where a pH4.3 acetate buffer was used. The simulated flue gas contained 11-12% O₂,and the total gas flow rates were 0.8-1.0 liter per minute. Totalexperimental time ranged between 2 and 3 hours. Other operatingconditions used, and the results obtained, are given in the Table below.

                                      TABLE                                       __________________________________________________________________________         NO  Temp.                                                                             Initial                                                                           Phosphorus                                                                          Maximum                                                                              Average                                         Example                                                                            (ppm)                                                                             (°C.)                                                                      pH  added (gm)                                                                          % Removal                                                                            % Removal                                       __________________________________________________________________________    4a    60 50  6.5 1.5   100    100                                             4b    65 50  6.3 0.8   100    100                                             4c    400                                                                              50  6.2 0.8    80    43                                              4d    430                                                                              50  7.4 1.5   100    76                                              4e   1950                                                                              50  6.2 3.1    55    29                                              4f   2000                                                                              75  4.3 4.0    95    72                                              __________________________________________________________________________

From these examples, it is seen that very efficient removal is achievedat 50° C. when low concentrations of NO are to be removed. In theexamples given, satisfactory removal of higher concentrations of NO wereachieved at 75° C.

Example 5

The simultaneous removal of NO and SO₂ from a simulated flue gas wascarried out using a yellow phosphorus emulsion mixed with a slurry oflimestone. The apparatus used in this experiment is similar to that ofExample 1, except that the reactor had a volume of about 1.2 liter (110mm i.d.×130 mm). 0.9 liters of an aqueous emulsion/slurry containing3.3% by weight of yellow phosphorus and 5.0% by weight of CaCO₃ wasdispersed by a magnetic stir bar. The temperature of the scrubbingliquor was kept at 55° C. and the pH was 7.5. The absorber was providedwith a 5 0% by weight slurry of CaCO₃. A simulated flue gas mixturecontaining 560 ppm NO, 2900 ppm SO₂, 10% O₂, and the balance N₂ wasbubbled into the slurry at a rate of about 1.3 liters per minute. Thereaction temperature was maintained at 55° C., whereas the pH of theslurry dropped from about 7.5 to about 4.2 after 3 hours. The removalrates of NO and SO₂ are shown in FIG. 5 wherein it is seen that theremoval of SO₂ quickly reaches about 100% and shortly thereafter theremoval rate of NO reaches about 100%. From these data, it appears thatNO removal by the use of yellow phosphorus is enhanced when SO₂ andlimestone are present.

The solid and liquid phases in the scrubber and in the absorber wereseparated by suction filtration and analyzed. The solid collected fromthe scrubber after the reaction was analyzed by laser Ramanspectroscopy; and was shown to contain CaSO₄.2H₂ O, in addition tounreacted CaCO₃ and yellow phosphorus. In the absorber downstream, onlyunreacted CaCO₃ was detected. No CaSO₃.1/2H₂ O precipitate was detectedin either the scrubber or the absorber.

It was found that both the scrubbing liquor and the absorbing solutioncontain NO₂ ⁻, NO₃ ⁻, SO₃ ⁼, SO₄ ⁼, H₂ PO₂ ⁻, H₂ PO₃ ⁻, and H₂ PO₄ ⁻.Since the amount of NO₂ ⁻ and NO₃ ⁻ recovered could account for onlyabout 50% of the NO absorbed and a substantial amount of HSO₃ waspresent in the scrubbing liquor, a search for nitrogen-sulfur compoundswas conducted. Indeed, we found that about 40% of the NO absorbed couldbe accounted for by the formation of the nitrogen-sulfur compoundshydrozylamine disulfonate (HADS) and amine disulfonate (ADS) in aslightly acidic (pH about 4) scrubbing liquor. We also found that bothHADS and ADS were subsequently hydrolyzed to NH₄ ⁺ in the scrubbingliquor when the pH was lowered to about 2. The formation ofnitrogen-sulfur compounds via the reaction of NO₂ ⁻ and HSO₃ ⁻ inscrubbing liquor and their hydrolysis reactions have been well studied,and the NH₄ ⁺ formation follows from these studies. Therefore the use ofyellow phosphorus emulsions for combined NO_(x) and SO₂ removal resultsin the conversion of undesirable NO to NH₄ ⁺, NO₃ ⁻, and NO₂ ⁻, all ofwhich are desirable chemicals for the manufacture of fertilizer.

Example 6

A simulated flue gas mixture with about 5% oxygen was prepared bypassing liquid nitrogen from a standard pressurized 160 liter dewarthrough a vaporizer column (Hex Industries) and by mixing the gas withcompressed air to obtain the desired oxygen concentration. NO and SO₂were blended in to give concentrations of 275-350 ppm and 1500-3000 ppm,respectively. CO₂ could be added up to approximately 10% of the totalgas flow. The gas stream flowed, at a rate of 20 acfm, through anelectric air heater where it was heated to a temperature of 350° F. Theheated gas then entered the absorber. Two types of absorbers weretested: a spray tower type and a bubbling type absorber. The spray towerabsorber was a 4 in diameter by 4 ft long glass column installed withspray nozzles (Spraying Systems, Inc.). Two different spray nozzleset-ups were tested: a two nozzles (2.0 gal/min per nozzle) in seriesset-up and a ten nozzle (0.2 gal/min per nozzle) set-up, in whichnozzles were divided into two parallel rows with each row containing 5nozzles in series. An aqueous mixture of P₄ and limestone slurry wassprayed in the absorber. A countercurrent flow of flue gas entered atthe base of the absorber and passed upward through the falling spray ofslurry as shown in FIG. 6.

The bubbling absorber was a scaled-down simulation of the Bechtel CT-121system. As shown in FIG. 7, the bubbling absorber system included aprescrubber and a scrubber. The spray tower column just described wasused as a prescrubber. The scrubber column was constructed of a 4 indiameter by 4 ft section stainless steel pipe. Four 5/8 in diameterstainless steel tubes served as impingers directing the gas into thelimestone slurry at the bottom of the column. An aqueous emulsion of P₄was sprayed downward in a prescrubber which quenched and conditioned theflue gas flowing upward. The pretreated flue gas then entered ascrubbing column downward through impingers that submerged about 10inches under the aqueous limestone slurry. A froth layer was formed whenthe gas entered the scrubber, which provided a greatly extendedinterfacial area for gas-liquid contact. Air (0.85 cfm) was fed into thebottom of the scrubber to force oxidize the HSO₃ ⁻ to SO₄ ²⁻. Probes inthe column allowed measurements of pH and temperature.

The concentration of P₄ in the scrubbing liquors ranged from 0.5 to 0.8%w/w, while that of limestone was 6-10% w/w. A 2-liter Erlenmeyer flaskwas used as a holding tank for liquid mixture from the spray column. Aliquid mixture was recirculated with a centrifugal pump (Price Pump Co.)to the top of the spray column. The pH of the scrubbing liquor wascontrolled by feeding an aqueous mixture of limestone and lime from athermostatted reservoir (50° C.) to the hold tank by a Masterflex pump(Randolph-Austin Corp.). The pH range studied was 3.5 to 6. The holdtank temperature was controlled at 50°-55° C. P₄ could be continuouslyfed into the system from a burette containing liquid P₄ and water. P₄(specific gravity 1.80) settled at the bottom of the burette. Theburette was wrapped with a heating tape to maintain the temperature ofP₄ in the burette above 44° C., its melting point.

The gas from the absorber was then directed through a washing column. Inthe washing column, concentrated phosphoric acid (40-60%) was sprayedthrough a 1 gal/min nozzle (Spraying Systems, Inc.) and recirculated bya centrifugal pump to absorb the phosphorus "white smoke". Thephosphoric acid "white smoke" was produced by oxidation of the P₄ tophosphorus pentaoxides which picked up moisture to form phosphoric acid"white smoke" aerosols.

The NO_(x) chemiluminescent analyzer and the SO₂ fluorescent analyzerhave intake connections to the gas stream at various points along thesystem. The NO, NO_(x) and SO₂ concentrations can thus be measured andthe effectiveness of the absorber operation can be evaluated.

Liquids from the different columns in the system can be analyzed by ionchromatography and laser Raman spectroscopy to determine the identityand concentration of the anions present. The solid precipitates can beanalyzed by FTIR and laser Raman spectroscopy.

At a flow rate of 20 acfm, the superficial velocity of flue gas in a 4in diameter column was about 4 ft/sec, which is typical in a CT-121scrubber. This is slower than that in spray tower systems, where thevelocity is 8-12 ft/sec. However, the gas-liquid contact time and liquidgas (L/G) ratio are more significant physical parameters to simulatewhen scaling down. In the case of a spray tower scrubber, the contacttime of gas and liquid sprays is about 2-5 sec and L/G ranges between 60and 120 depending on the SO₂ concentrations and removal requirements. Inthe case of a CT-121 scrubber, the SO₂ removal efficiency is a functionof the depth of submergence of the spargers. A submergence of 8 incheswill generally provide 90% removal efficiency with a gas superficialvelocity of 4 ft/sec. A 10-inch submergence was provided in the testequipment. The height of froth layer created in a 4 inch column issomewhat larger than that in a commercial reactor, however. The mainobjective of the small bench-scale test was to prove the concept ofNO_(x) removal simultaneously with SO₂ removal in wet limestone systems,and not to obtain data for scale-up to a commercial size.

The results of a typical run on the removal efficiency of NO and SO₂ isshown in FIG. 8. This was a run using a bubbling absorber. An aqueousemulsion of P₄ initially containing 0.8% w/w P₄ was sprayed andrecirculated in the prescrubber. The initial limestone concentration inthe bubbling scrubber was 6% w/w and the temperature of the limestoneslurry was 55° C. The flue gas contained 300 ppm NO, 1500 ppm SO₂ and4.5% O₂. The flow rate of flue gas was 15.60 acfm, corresponding to asuperficial velocity (V_(f)) of 3.3 ft/sec in the column. The removalefficiency of NO could be maintained at more than 85% during most of theexperiment until near the end of the run, when the concentration of P₄was substantially depleted. Also, the initial removal efficiency of NOwas not as good. This is attributed to the poor mixing of P₄ with waterat the beginning of the experiment. The spray nozzles can break up P₄globules and create a finely dispersed P₄ emulsion in water. The removalefficiency of SO₂ depends strongly on the pH of the scrubbing liquor.Initially, SO₂ was removed completely at a pH of 5.5. The efficiencydropped to about 90% when the pH of slurry decreased to 4.5.

The NO removal efficiency measures the effectiveness of NO_(x)absorption in the scrubbing liquor, and depends on the extent of NOoxidation to NO₂, the mixing of flue gas with liquor, andsulfite/bisulfite ion concentration. The oxidation efficiency measuresthe effectiveness of the oxidation of NO to NO₂ by the P₄ -inducedoxidation method. As stated previously, the NO oxidation efficiency isrelated to the concentration of P₄ in the spray liquor, O₂ concentrationin the flue gas, temperature, and the mixing of the spray with flue gas.The factors influencing the mixing include the L/G ratio, size anduniformity of the spray, and the contact time.

Example 7

A set of experiments were conducted by varying the flow rate of flue gasat a constant flow rate of recycling liquor. As a result, thesuperficial velocity and contact time of the flue gas with the sprayalso varied. The apparatus and conditions, unless otherwise stated, weresimilar to those of Example 6. The resulting NO oxidation and removalefficiencies as a function of L/G and P₄ concentration are shown inFIGS. 10 and 11 for a spray tower and a bubbling scrubber, respectively.With a spray tower absorber, an aqueous emulsion of P₄ and limestone wassprayed and recirculated in a single spray column. Therefore, thegeneration of O₃, the oxidation of NO to NO₂ and the absorption of NO₂and SO₂ in scrubbing liquor took place in one column. The oxidationefficiency was more than 80% at a L/G of 60, while the removalefficiency was only 60%. The removal efficiency did not reach 80% untila L/G of 90. The increase of P₄ concentration from 0.5% to 0.8% improvedslightly both the oxidation and removal efficiencies. The effect wasmore apparent at low L/G values. With a CT-121 type scrubber, theoxidation occurred in a prescrubber where an aqueous emulsion of P₄ wassprayed, and the absorption took place in a bubbling absorber containinga limestone slurry. The oxidation efficiencies were more than 90% andthe removal efficiencies more than 80% at a L/G of 60 or more. Theseresults are better than those with a spray tower scrubber at givenexperimental conditions. This is attributed mainly to the difference inspray quality between two types of scrubbers. The spray nozzles aresusceptible to clogging when the recirculating liquor contains limestoneand gypsum particles. The limestone in the spray may also surround theP₄ droplets and reduce the effective concentration of P₄.

Because the diameter of the spray column is 4 in, the droplets hit thewall in a short distance after being sprayed. The liquor then flows downalong the wall of the column and exhibits poor contact with flue gas.Consequently, the mixing in the bench-scale system is not as effectiveas that in a commercial scale system at a given L/G. The considerationof the contact time of the droplets with flue gas may be moremeaningful. FIG. 11 shows a plot of the NO oxidation and removalefficiencies as a function of contact time. The results were obtainedwith a spray tower scrubber. The gas-droplet contact distance wasestimated to be 2 ft. The contact time can be varied by changing theflow rate of the flue gas. The NO oxidation achieved 100% efficiency,and the NO_(x) removal reached 90% efficiency with a contact time of 1.4secs, which is less than that (2-5 secs) in a commercial system.

Example 8

The NO oxidation efficiency as a function of contact time in aprescrubber of a CT-121 simulation system was carried out. A plot of theresults is shown in FIG. 12. The apparatus was similar to the one usedin Example 7. The spraying liquor was composed of an aqueous emulsion ofP₄ and did not contain limestone. The spray appeared to be more uniformand the nozzles did not show clogging problems. The contact distance wasestimated to be 2.5 ft. The oxidation efficiency was slightly betterthan that in a spray tower system at the same contact time, but theimprovement was less than the experimental uncertainty.

Based upon these experiments, a possible commercial conceptual processconfiguration with the following features is shown in FIG. 13.

injecting a phosphorus emulsion into an existing wet limestone scrubbingsystem

adding a "Brink" separator/hydrator downstream of the scrubber tocapture and convert the P₂ O₅ to phosphoric acid byproduct

installing necessary equipment to recover other byproducts (calciumphosphate and ammonium phosphate)

adding new fan capacity to compensate for the additional pressure drop

The actual P/NO requirement depends on the equipment (i.e. scrubber)used for contacting the gas and the phosphorus emulsion. In thebench-scale equipment used where the contacting time was short and themixing was relatively inefficient, the required P/NO ratio was from 0.6to 1.0. For more efficient contacting devices and longer contact time,as typically in most commercial scrubbers, a ratio around 0.5 can bereasonably expected.

While only illustrative embodiments have been described, it will beappreciated that various modifications may be made, and the invention isto be limited only by the spirit and scope of the appended claims.

What is claimed is:
 1. A method of treating exhaust gases containingacid forming pollutants including NO and obtaining a phosphoric acidproduct which comprises the steps of contacting the exhaust gas with anaqueous emulsion or suspension of yellow phosphorus, oxidizing thephosphorus while in contact with said exhaust gases so as to generatephosphoric acid and oxidize the NO to NO₂, removing the NO₂ andrecovering the phosphoric acid.
 2. The method of claim 1 wherein theexhaust gas has a contact time of about 0.05 to about 3.5 seconds withthe phosphorus emulsion or suspension when the contact occurs in abubbling type absorber.
 3. The method of claim 1 wherein the exhaust gashas a contact time of about 0.5 to about 10 seconds with the phosphorusemulsion or suspension when the contact occurs in a spray tower typeabsorber.
 4. The method as defined in claim 1, wherein the temperatureof the aqueous emulsion or suspension is from about 20° C. to about 95°C.
 5. The method as defined in claim 1, wherein the temperature of theaqueous emulsion or suspension is from about 45° C. to about 75° C. 6.The method as defined in claim 1, wherein the amount of yellowphosphorus in the emulsion or suspension is from about 0.01 to about20.0% by weight.
 7. The method of treating exhaust gases as defined inclaim 1, wherein the amount of yellow phosphorus in the emulsion orsuspension is from about 0.1 to about 10.0% by weight.
 8. The method asdefined in claim 1, wherein the pH of the emulsion or suspension fromabout 0 to about
 7. 9. The method as defined in claim 1, wherein theamount of oxygen in the flue gas being treated is between about 1% andabout 16% by volume.
 10. The method as defined in claim 9, wherein theoxygen content is adjusted by adding air to the exhaust gas.
 11. Amethod of obtaining phosphoric acid from the treatment of exhaust gasescontaining NO_(x) and SO₂, which comprises the steps of contacting theexhaust gases with an aqueous emulsion or suspension of yellowphosphorus and an amount of alkali sufficient to provide and retain a pHwithin the aqueous emulsion or suspension between about 3 and 9,oxidizing said phosphorus while in contact with said exhaust gas togenerate P₄ O₁₀ and O, allowing said P₄ O₁₀ to associate with H₂ O toyield phosphoric acid and recovering said phosphoric acid.
 12. Themethod of claim 11 wherein the exhaust gas has a contact time of about0.05 to about 3.5 seconds with the phosphorus emulsion or suspensionwhen the contact occurs in a bubbling type absorber.
 13. The method ofclaim 11 wherein the exhaust gas has a contact time of about 0.5 toabout 10 seconds with the phosphorus emulsion or suspension when thecontact occurs in a spray tower type absorber.
 14. The method as definedin claim 11 wherein the aqueous emulsion or suspension is maintained ata temperature of about 20° C. to about 95° C.
 15. The method as definedin claim 11, wherein the aqueous emulsion or suspension is maintained ata temperature of about 45° C. to about 75° C.
 16. The method of treatingexhaust gases as defined in claim 11, wherein the alkali comprisescalcium carbonate.
 17. The method of treating exhaust gases as definedin claim 11, wherein the alkali comprises ammonia.
 18. A method oftreating exhaust gases containing initially from about 15 to about 3000parts per million NO_(x), which comprises the steps of providing anaqueous emulsion of yellow phosphorus wherein the amount of thephosphorus is from about 0.01 to about 20.0% by weight, and thetemperature of the emulsion is between about 20° C. and about 95° C.,and contacting the gas with the emulsion for a time sufficient togenerate phosphoric acid and to reduce the concentration of NO_(x) inthe exhaust gas by about 20% up to about 95% of the initialconcentration and for a time not sufficient to absorb a substantialportion of said generated phosphoric acid.
 19. The method as defined inclaim 18, wherein the exhaust gas also contains from about 100 to about3,500 parts per million sulfur dioxide.
 20. The method as defined inclaim 19, wherein the aqueous emulsion also contains alkali.
 21. Themethod as defined in claim 18, wherein the aqueous emulsion alsocontains calcium carbonate.
 22. A method of treating exhaust gasescontaining initially from about 15 to about 3000 parts per millionNO_(x) and from about 100 to about 3,500 parts per million SO₂, whichcomprises the steps of:(a) providing an aqueous emulsion of yellowphosphorus wherein the amount of the phosphorus is from about 0.01 toabout 20.0% by weight, and the temperature of the emulsion is betweenabout 20° C. and about 95° C.; (b) contacting the gas with the emulsionfor a time sufficient to oxidize said phosphorus to yield P₄ O₁₀ with O,to generate phosphoric acid by contacting said P₄ O₁₀ with H₂ O and fora time not sufficient to absorb a substantial portion of said generatedphosphoric acid; (c) further providing a solution or a second emulsionor suspension wherein said solution, or second emulsion or suspensioncontains an alkali; and (d) contacting the solution or emulsion orsuspension from step (c) with the exhaust gas from step (b) and asubstantial portion of the phosphoric acid from step (b) for a timesufficient to reduce the concentration of SO₂ in the exhaust gas byabout 40% to about 98% and to reduce the concentration of NO_(x) in theexhaust gas by about 20% up to about 95% of the initial NO_(x)concentration and for a time not sufficient to absorb a substantialportion of the phosphoric acid.
 23. A method of treating exhaust gasescontaining initially from about 15 to about 1000 parts per million NO,which comprises the steps of providing an aqueous emulsion of yellowphosphorus wherein the amount of phosphorus is from about 0.1 to about5.0% by weight, and the temperature of the emulsion is from about 45° C.to about 75° C., and passing the gas through the emulsion in directcontact therewith with the contact time being sufficient to oxidize saidphosphorus to yield P₄ O₁₀ and O, to generate phosphoric acid from saidP₄ O₁₀, and to reduce the concentration of NO in the exhaust gas byabout 20% up to about 98% of the initial concentration and not beingsufficient to absorb a substantial portion of said phosphoric acid. 24.A method of treating exhaust gases containing initially from about 15 toabout 1000 parts per million NO_(x), which comprises the steps ofproviding an aqueous emulsion of yellow phosphorus wherein the amount ofphosphorus is from about 0.1 to about 5.0% by weight, and thetemperature of the emulsion is from about 45° C. to about 75° C., andpassing the gas through the emulsion in direct contact therewith withthe contact time being sufficient to oxidize said phosphorus to yield P₄O₁₀ and O, to generate phosphoric acid from said P₄ O₁₀, and to reducethe concentration of NO_(x) in the exhaust gas by about 20% up to about95% of the initial concentration and not being sufficient to absorb asubstantial portion of said phosphoric acid.
 25. A method of treatingexhaust gases containing initially from about 15 to about 1,000 partsper million NO and from about 100 to about 3,000 parts per million SO₂,which comprises the steps of:(a) providing an aqueous emulsion of yellowphosphorus wherein the amount of phosphorus is from about 0.1 to about5.0% by weight, and the temperature of the emulsion is from about 45° C.to about 75° C.; (b) passing the gas through the emulsion in directcontact therewith with the contact time being sufficient to oxidize saidphosphorus to yield P₄ O₁₀ and O, to generate phosphoric acid from saidP₄ O₁₀, and to reduce the concentration of NO in the exhaust gas byabout 20% up to about 95% of the initial concentration and not beingsufficient to absorb a substantial portion of said phosphoric acid; (c)further providing a solution or a second emulsion or suspension whereinsaid solution or second emulsion or suspension contains an alkali; and(d) passing the exhaust gas from step (b) and a substantial portion ofthe phosphoric acid from step (b) through the solution, emulsion orsuspension from step (c) in direct contact therewith, the contact timebeing sufficient to reduce the concentration of SO₂ in the gas by about40% to about 98% of the initial concentration and not being sufficientto absorb a substantial portion of said phosphoric acid.
 26. A method ofobtaining a phosphoric acid product from the treatment of exhaust gasescontaining acid forming pollutants including NO which comprises thesteps of contacting the exhaust gases, an aqueous emulsion or suspensionof yellow phosphorus and O₂ with each other to oxidize NO to NO₂ and toyield phosphoric acid, recovering the phosphoric acid and removing saidNO₂.
 27. A method of treating exhaust gases containing initially fromabout 15 to about 3000 parts per million NO_(x), which comprises thesteps of providing an aqueous emulsion of yellow phosphorus wherein theamount of the phosphorus is from about 0.01% to about 0.5% by weight,and the temperature of the emulsion is between about 20° C. and about95° C., and contacting the gas with the emulsion for a time sufficientto oxidize the NO in the exhaust gas and reduce the concentration ofNO_(x) in the exhaust gas from about 20% up to about 95% of the initialconcentration and to generate phosphoric acid but not to absorb asubstantial portion of the generated phosphoric acid.
 28. A method oftreating exhaust gases containing initially from about 15 to about 1000parts per million NO, which comprises the steps of providing an aqueousemulsion of yellow phosphorus wherein the amount of phosphorus is fromabout 0.01% to about 0.5% by weight, and the temperature of the emulsionis from about 45° C. to about 75° C., and passing the gas through theemulsion in direct contact therewith with the contact time beingsufficient to oxidize said phosphorus to yield P₄ O₁₀ and O, to generatephosphoric acid from said P₄ O₁₀, and to oxidize the NO and reduce theconcentration of NO in the exhaust gas to less than 10 ppm and not beingsufficient to absorb a substantial portion of the phosphoric acid.